Investigation on the Potential Relationships between Geophysical Properties and CH4 Adsorption in a Typical Shale Gas Reservoir

The Longmaxi Formation is considered to be a potential shale gas reservoir in Sichuan Basin of China. In this work, 28 typical shale core samples are collected from the Longmaxi Formation and a comprehensive characterization of the geophysical and petrochemical properties of these core samples is obtained. Thermogravimetric method is applied to measure the excess CH4 adsorption on these shale samples; the adsorbed CH4 density is then calculated using the grand canonical Monte Carlo simulation method, which is consequently used to determine the absolute CH4 adsorption. Based on the converted absolute adsorption, the Langmuir adsorption model is employed to determine the maximum adsorption capacity of CH4 on these shale samples. The inter-relationships between the adsorption capacity of CH4 and the geophysical properties of the 28 Longmaxi shale cores are analyzed. The geophysical and petrochemical characterization results show that the Longmaxi Formation generally possesses pores with diameter in the range of 1-10 000 nm, while mesopores (i.e., 2-50 nm) account for the most proportion. The Longmaxi shale has a complex mineral composition, wherein clay minerals constitute more than half of the total minerals. The maximum adsorption capacity of CH4 on these typical shale increases as the burial depth increases, indicating a higher potential for the commercial development of the Longmaxi shale with deep formations; in addition, the maximum adsorption capacity has a positive and linear relationship with the total organic carbon content. The CH4 adsorption presents linear and positive inter-relationships with clay minerals, i.e., illite and montmorillonite, other than kaolinite.